1. Field of the Invention
This invention relates to a compact magnetometer element comprised of fiber glass and superconducting thin-film components on silicon, and a support structure used for integration of the individual magnetometer elements into a detector array applicable to magnetoencephalographic (MEG) recordings of human brain activity. Such devices are used for recording the weak, space and time dependent magnetic fields arising from neural activity. In medical research and diagnostics these methods are gaining more and more attention. Especially, the study of the brain function and malfunction in man can be done noninvasively with this method, i.e. without touching the subject or exposing him to electromagnetic radiation or radioactive tracers. The essential advantage of the MEG method as compared to the widely used electroencephalographic (EEG) method, i.e. measurement of the electric potential on the scalp, is due to the fact that the nonuniform conductivity of the human tissue distorts the magnetic signals of neural origin much less than the associated electric potential distributions on the scalp. Consequently, by the MEG method it is possible to locate the source currents associated with the brain activities with a spatial and temporal resolution of a few millimeters and milliseconds. The method has been described in more detail for example in CRC Critical Reviews in Biomedical Engineering, vol. 14 (1986), number 2, pp. 93-126.
Practical MEG devices must be able to detect magnetic signals corresponding to flux densities of the order of 100 fT or below. In addition, the field must be measured simultaneously at several, up to hundred, different locations around the skull. The only technical device possessing a sensitivity sufficient for the measurement of these signals is the so called Superconducting Quantum Interference Device (SQUID) magnetometer. A modern SQUID with the associated signal coils is fabricated on a polished silicon substrate by using thin film technique widely used in the fabrication of integrated circuits (see for example Superconducting Quantum Interference Devices and their Applications, eds H. D. Hahlbohm and H. Lubbig, Walter de Gruyter, Berlin 1985, pp. 729-759). The principle and function of a SQUID magnetometer has been described in detail for example in Journal of Low Temperature Physics, vol. 76 (1989), number 56, pp. 287-386. These devices work only in a very low ambient temperature. Typically, the device is immersed in liquid helium contained in a vacuum insulated dewar vessel. The working temperature of the SQUID is then 4.2 degrees Kelvin.
The present invention is directed to a SQUID magnetometer element having a novel type of mechanical construction, and a support structure to be used in a helium dewar for integration of the individual magnetometer elements into an array that covers the entire cranium of a human subject. Such MEG devices, collecting practically all the information available through the method, have not been constructed so far. A magnetometer insert on which the individual magnetometer elements and their support structure described here can be mounted is described in a copending patent application "A multichannel device for measurement of weak spatially and temporally varying magnetic fields" by Ahonen, Knuutila, Simola, and Vilkman, Ser. No. 07/807,149, filed Dec. 13, 1991 now, U.S. Pat. No. 5,243,281, issued Sep. 7, 1993.
2. Description of the Related Art
In constructing the magnetometer element one must be able to join together the substrate of the thin film SQUID, a piece of thin silicon wafer about one inch in diameter, and a base element made of insulating material and containing the electrical contacts and the structures needed for mechanical mounting of the magnetometer element on the support structure. The joint between the silicon and the base element must sustain the repeated thermal cycling between room temperature and the liquid helium temperature during the testing and maintenance of the magnetometer. Because of the relatively low thermal expansion of silicon such a joint made by gluing is not reliable. It is also impossible to mount commercially available connectors directly and reliably on silicon.
The signal coil on the magnetometer element must be located as close to the bottom of the helium dewar as possible and the element itself must be flat. The first requirement arises from the need to minimize the distance between the detector coil and the source for the magnetic field located in the brain outside the dewar. This is necessary because the amplitude of the measured signal is inversely proportional to the third power of the distance from the source. The latter requirement is explained by the need to minimize the diameter of the neck through which the magnetometer array is introduced into the dewar. This minimizes the boil off rate of liquid helium which is an essential problem in the construction of dewars for MEG magnetometers. To cover the whole skull one must place magnetometer channels on opposite sides of the skull. The minimum inner diameter of the dewar neck is therefore approximately 25 centimeters added with the heights of the two magnetometer elements on opposite sides of the head. In the prior art devices the height of the magnetometer elements along with the structures needed for their mounting has been several centimeters. The boil off rate in dewars with such wide necks is dominated by the conduction of heat along the neck, and reducing the diameter of the required neck , i.e. The height of the magnetometer elements, is therefore crucial.
Other ways to avoid a large neck would be 1) to construct the dewar in such a way that the magnetometer insert is not introduced into the dewar through the neck, but it is rather built in the dewar permanently when fabricating the dewar (EP 200 958), or 2) to construct the vacuum of the dewar so that it can be opened (see for example Advances in Biomagnetism, eds S. J. Williamson, M. Hoke, G. Stroink and M. Kotani, Plenum, New York 1989, pp. 677-679). The first alternative makes the service of the detector coils difficult or even impossible. The SQUIDs at least need maintenance, and should therefore be mounted on an insert that can be taken out from the dewar. In this case one would have to use, between the SQUIDs and the detector coils, superconducting multicontact connectors which are not reliable enough. The second alternative, a dewar having a narrow neck for transfer siphon only but provided with a large cold vacuum seal, is potentially dangerous. These dewars are used in small, relatively closed magnetically shielded rooms in the presence of possibly disabled neurological patients. This implies that breaking of the vacuum seal might lead to severe consequences. Therefore, the MEG dewars must be manufactured by using conventional, reliable techniques.
The central goal in both scientific and clinical use of MEG devices is to locate the cortical source currents responsible for the measured neuromagnetic field as accurately as possible. This goal can be achieved only if 1) the field is measured over the entire cortex and if 2) the geometry of the measuring device and its location with respect to the brain is known accurately. To cover the appropriate parts of the skull one needs a support structure for the magnetometer array which is roughly hemispherical in shape. The diameter of the hemisphere should be about 25 cm and the accuracy of its overall geometry better than a millimeter which roughly corresponds to the relevant accuracy in locating neurological current sources. Especially, one must know the size, shape, and location of the magnetometer array when it is at liquid helium temperature in the dewar.